Wednesday, 26 September 2018

JavaScript modules, export and import

Let's consider web app project root directory which contains the following files:


Without modules:


function clearElement(id) {...}
function log(text) {...}


function foo() {

In HTML file scripts have to be added in order so functions, objects, or primitive values from previously included scripts are visible in scripts that use them:


         <script src='/scripts/common.js'></script>
         <script src='/scripts/array_demo.js'></script>

This can be a problem in case when there are many and/or complex dependencies. The solution is using ECMAScript modules. This will work in HTML5-compliant browsers.

With modules:

What is the difference between a common (classic) JavaScript script and a module?

A classic script is just a standard JavaScript script as you know it. A module script is one that contains an ES6 module, i.e. it uses (or: can use) import and export declarations. [source]


function clearElement(id) {...}
function log(text) {...}

export {


import { clearElement, log} from './common.js';

function foo() {


         <script type="module" src='/scripts/array_demo.js'></script>

export and import statements are defined in ECMAScript 2015 (6th Edition, ECMA-262).

module script type is available only in HTML5-compliant browsers [source].

Modules VS Require.JS

Require.JS "modularizes" JS scripts and basically emulates import/export functionality. With ES6 modules available there is no need to use Require.JS.

Further reading:

ECMAScript modules in browsers
ES6 In Depth: Modules
HTML/Element/script (MDN)
ECMAScript 6 modules: the final syntax
export (MDN)
import (MDN)
CommonJS vs AMD vs RequireJS vs ES6 Modules

How to have two pages with same titles withing the same space in Confluence

If you try to create and save a page in Confluence which has the same title as some other page in the same space, you'll get this error:

This is because it is NOT possible to have multiple pages with the same name in a single space. Workarounds include prepending page name with (shortened) parent page name or adding invisible Unicode characters.


Tuesday, 25 September 2018

ESLint VSCode plugin

If we install ESLint plugin for VSCode but not ESLint package itself, we'll get this error in VSCode:

In my post Introduction to ESLint I followed the ESLint guide for its local installation which instructs creating its config file in node_modules/.bin. This might not be the optimal (or better to say, working) solution if we want to use ESLint plugin for VSCode.

I installed that plugin and opened a directory of a test web application which contains JS files and has locally installed eslint. .eslintrc.json was created in .\node_modules\.bin. After couple of seconds upon opening project directory I got this error in ESLint output:

[Info  - 3:18:08 PM] ESLint server stopped.
[Info  - 3:18:08 PM] ESLint server running in node v8.9.3
[Info  - 3:18:08 PM] ESLint server is running.
[Info  - 3:18:11 PM] ESLint library loaded from: c:\dev\github\demo-html-css-js\node_modules\eslint\lib\api.js
[Error - 3:18:11 PM]
Failed to load plugin react: Cannot find module 'eslint-plugin-react'
Happened while validating C:\dev\github\demo-html-css-js\scripts\objects_demo.js
This can happen for a couple of reasons:
1. The plugin name is spelled incorrectly in an ESLint configuration file (e.g. .eslintrc).
2. If ESLint is installed globally, then make sure 'eslint-plugin-react' is installed globally as well.
3. If ESLint is installed locally, then 'eslint-plugin-react' isn't installed correctly.

Consider running eslint --debug C:\dev\github\demo-html-css-js\scripts\objects_demo.js from a terminal to obtain a trace about the configuration files used.

I found here that global .eslintrc located in my user directory could be the cause of this. And indeed, I found c:\Users\User\.eslintrc with the following content:

/* See all the pre-defined configs here: */
"extends": "defaults/configurations/eslint",
"parser": "babel-eslint",
"ecmaFeatures": {
"jsx": true
"plugins": [
"env": {
"amd": true,
"browser": true,
"jquery": true,
"node": true,
"es6": true,
"worker": true
"rules": {

"eqeqeq": 2,
"comma-dangle": 1,
"no-console": 0,
"no-debugger": 1,
"no-extra-semi": 1,
"no-extra-parens": 1,
"no-irregular-whitespace": 0,
"no-undef": 0,
"no-unused-vars": 0,
"semi": 1,
"semi-spacing": 1,
"valid-jsdoc": [
{ "requireReturn": false }

"react/display-name": 2,
"react/forbid-prop-types": 1,
"react/jsx-boolean-value": 1,
"react/jsx-closing-bracket-location": 1,
"react/jsx-curly-spacing": 1,
"react/jsx-indent-props": 1,
"react/jsx-max-props-per-line": 0,
"react/jsx-no-duplicate-props": 1,
"react/jsx-no-literals": 0,
"react/jsx-no-undef": 1,
"react/jsx-sort-prop-types": 1,
"react/jsx-sort-props": 0,
"react/jsx-uses-react": 1,
"react/jsx-uses-vars": 1,
"react/no-danger": 1,
"react/no-did-mount-set-state": 1,
"react/no-did-update-set-state": 1,
"react/no-direct-mutation-state": 1,
"react/no-multi-comp": 1,
"react/no-set-state": 0,
"react/no-unknown-property": 1,
"react/react-in-jsx-scope": 0,
"react/require-extension": 1,
"react/self-closing-comp": 1,
"react/sort-comp": 1,
"react/wrap-multilines": 1

I moved my .eslintrc.json from node_modules\.bin\ to the project's root and added in it:

"root": true

After this I re-opened project's workspace in VSCode and ESLint didn't report any errors and worked with no issues.

How to disable ESLint globally and per-project?

Introduction to ESLint

What is ESLint?

JavaScript linting utility

What is linting?

Code linting is a type of static analysis that is frequently used to find problematic patterns or code that doesn’t adhere to certain style guidelines.

Why do we need linting in JavaScript?

JavaScript, being a dynamic and loosely-typed language, is especially prone to developer error. Without the benefit of a compilation process, JavaScript code is typically executed in order to find syntax or other errors. Linting tools like ESLint allow developers to discover problems with their JavaScript code without executing it.


ESLint community is advocating local installation although a global installation is used in the guide on ESLint's webpage [source].

"If you want to include ESLint as part of your project’s build system, we recommend installing it locally." [Local Installation and Usage]

To install ESLint locally (after running npm init):

> npm install eslint --save-dev

package.json now contains:

"devDependencies": {
  "eslint": "^5.6.0"


To set up configuration file use:

C:\dev\github\demo-html-css-js\node_modules\.bin>eslint --init
? How would you like to configure ESLint? (Use arrow keys)
  Use a popular style guide
> Answer questions about your style
  Inspect your JavaScript file(s)

NOTE: Read here why it might be better to run this command and create config file in the root directory of your project. If you choose to do so, navigate to root directory and run npx eslint --init.

This option makes eslint asking a set of questions:

? How would you like to configure ESLint? Answer questions about your style
? Which version of ECMAScript do you use? ES2018
? Are you using ES6 modules? Yes
? Where will your code run? Browser
? Do you use CommonJS? No
? Do you use JSX? No
? What style of indentation do you use? Spaces
? What quotes do you use for strings? Single
? What line endings do you use? Windows
? Do you require semicolons? Yes
? What format do you want your config file to be in? JSON
Successfully created .eslintrc.json file in C:\dev\github\demo-html-css-js\node_modules\.bin

This config file looks like this:

..\node_modules\.bin>type .eslintrc.json
    "env": {
        "browser": true,
        "es6": true
    "extends": "eslint:recommended",
    "parserOptions": {
        "ecmaVersion": 2018,
        "sourceType": "module"
    "rules": {
        "indent": [
        "linebreak-style": [
        "quotes": [
        "semi": [


We can now call ESLint from command line:

..\node_modules\.bin> eslint <path_to_JavaScript_file>


How to disable “unexpected console statement” in Node.js?
disallow the use of console (no-console)

Introduction to Node Package Manager (npm)

What is npm?

  • package manager
  • task runner that can serve as a replacement for Gulp

How to install npm?

Upon installation user variable Path (in environment variables) gets a new entry:

How to see command line arguments?

How to check its version?

How to update npm?

Try the latest stable version of npm | npm Documentation

To verify it:

On Linux:

npm -v

npm install -g npm@latest
/home/bojan/.nvm/versions/node/v14.1.0/bin/npm -> /home/bojan/.nvm/versions/node/v14.1.0/lib/node_modules/npm/bin/npm-cli.js
/home/bojan/.nvm/versions/node/v14.1.0/bin/npx -> /home/bojan/.nvm/versions/node/v14.1.0/lib/node_modules/npm/bin/npx-cli.js
+ npm@6.14.5
updated 5 packages in 9.234s

npm -v


See here a complete definition of the package.

Node applications usually use (or depend on) multiple packages. There are three types of packages and each of them is listed within the object with the same name in packages.json:

  • regular (dependencies) - used in development in production
  • development (devDependencies) - packages are used only during application development and testing; we don't want to include them in the production and make users of our app unnecessarily download and build them 
  • optional (optionalDependencies) - dependencies which are used optionally - if they are found; their absence does not make application to fail


npm gets its config settings from:

  • command line
  • environment variables
  • npmrc files
  • package.json file (in some cases)


Global (per machine) 

Location: C:\Users\user\AppData\Roaming\npm\etc\npmrc

To find its location use:

>npm config get globalconfig


Location:  %USERPROFILE%\.npmrc

To find its location use:

>npm config get userconfig

Local (per project) 

Location: in project's root directory.

It defines where can npm look for and fetch packages - package registries by listing their URLs:

# use npmjs registry by default

# use xyz registry for packages in @xyz scope

After we change registry URL in .npmrc we need to do the following to force setting resolved field for all packages in packages-lock.json to this new domain:

  • delete node_modules
  • delete package-lock.json
  • run npm cache clean -f
  • run npm install


It lets npm know what the name of your package is as well as what dependencies it uses.

It is created and initialized via npm init command. This can be done retroactively - npm init can be executed for the already existing will only add to it package.json file.

It is a manifest of your project that includes the packages and applications it depends on, information about its unique source control, and specific metadata like the project's name, description, and author. [source]

The biggest reason for using package.json to specify a project’s dependencies is portability. For example, when you clone someone else’s code, all you have to do is run npm i in the project root and npm will resolve and fetch all of the necessary packages for you to run the app. [source]

All modules from package.json are installed to ./node_modules/. [source] This is npm install's default behavior. [source]

npm opens and reads package.json from the current directory. If it can't find it, it issues an error like:

Working with package.json
package.json - Specifics of npm's package.json handling
Semantic Versioning
npm semver calculator
What's the difference between tilde(~) and caret(^) in package.json?



If within the project we have some tool we want to call frequently e.g. after every code change we don't want to type it every time but want to automate the process by adding a command within scripts object. See Babel example.

-w instructs npm to watch for changes in the src folder. Every time you make a change to a file in src, this command is automatically executed.


Specifies alternative files to load in case bundling is done for the browser.

It is provided by a module author as a hint to javascript bundlers or component tools when packaging modules for client side use. [source]


"browser": {
 "vue": "vue/dist/vue.min.js"


package-lock.json is automatically generated for any operations where npm modifies either the node_modules tree, or package.json. [package-lock.json - A manifestation of the manifest]

Is there a way to force npm to generate package-lock.json?
Shall package-lock.json be added to version control?

CLI commands

alias: login

>npm login
Username: bojan
Email: (this IS public)

Logged in as bojan on

If logging for the first time and if %USERPROFILE%/.npmrc does not already exist, it will be created with the content similar to this one:


Used to add, list, or clean the npm cache folder.[npm-cache]
Try clearing the npm cache

cache clean

Cleans the npm cache.  Deletes all data out of the cache folder.

used to update and edit the contents of the user and global npmrc files.
config get

npm ci (named after Continuous Integration) installs dependencies directly from package-lock.json and uses package.json only to validate that there are no mismatched versions. If any dependencies are missing or have incompatible versions, it will throw an error.  
Use npm ci if you need a deterministic, repeatable build. For example during continuous integration, automated jobs, etc. and when installing dependencies for the first time, instead of npm install.  [continuous integration - What is the difference between "npm install" and "npm ci"? - Stack Overflow]

npm ci don't use the package.json to install modules it uses the package-lock.json file. This ensures reproducible builds—you are getting exactly what you expect on every install.  [NPM CI is better than NPM install in your CI/CD - DEV

Creates package.json. It is used when creating/initializing a package (not when installing it).

Installs in the local node_modules folder all dependencies listed in package.json.

npm install reads package.json to create a list of dependencies and uses package-lock.json to inform which versions of these dependencies to install. If a dependency is not in package-lock.json it will be added by npm install.  
Use npm install to add new dependencies, and to update dependencies on a project. Usually, you would use it during development after pulling changes that update the list of dependencies but it may be a good idea to use npm ci in this case.  [source]

node.js - Why does "npm install" rewrite package-lock.json? - Stack Overflow 

install --production
npm will not install modules listed in devDependencies. Same applies when --production is omitted but the NODE_ENV environment variable is set to production.

install -g npm
Updates npm package itself.

install --save-dev package1 package2...
Installs packages locally (in  project's devDependencies)

Lists all packages installed locally (for the current project) and also their dependencies.

list --depth=0
Lists all packages installed locally but without their dependencies.


To check the version of locally installed package we can use:

$ npm list  | grep axis*
├─┬ axios@0.19.2


$ npm list --depth=0  | grep axis*
├── axios@0.19.2

list -g
Lists all packages installed globally and also full path to the installation directory.
On Windows, that directory is: C:\Users\user\AppData\Roaming\npm.


list -g --depth=0
Lists globally installed packages but not their dependencies.


see adduser

run script_name
run-script script_name
Runs an arbitrary command (script) from a package's scripts key (scripts object in package.json). Optional argument: command. If no command is provided, it will list the available scripts (all properties of "scripts" object). [npm-run-script]

npm run allows you to pass arguments to the script you run by putting them after a double dash:
npm run script_name -- --arg1 --arg2

Lists installed packages with their currently installed versions, highest versions they can be upgraded to (this info is deducted from semver info for each package in package.json) and the latest version currently available in public package repository.

$ npm outdated
Package            Current    Wanted   Latest  Location
@types/flat         0.0.28    0.0.28    5.0.0  my-node-project
@types/jest        24.0.21    24.9.1   25.2.1  my-node-project
@types/node        11.15.2  11.15.12  13.13.4  my-node-project
axios               0.18.1    0.18.1   0.19.2  my-node-project
dotenv               7.0.0     7.0.0    8.2.0  my-node-project
flat                 4.1.0     4.1.0    5.0.0  my-node-project
https-proxy-agent    3.0.1     3.0.1    5.0.0  my-node-project
jest                24.9.0    24.9.0   25.5.3  my-node-project
pg                  7.12.1    7.18.2    8.0.3  my-node-project
ts-jest             24.1.0    24.3.0   25.4.0  my-node-project
tslib               1.10.0    1.11.1   1.11.1  my-node-project
tslint              5.20.0    5.20.1    6.1.2  my-node-project
typescript           3.6.4     3.8.3    3.8.3  my-node-project

Wanted values are deducted from semver info in package.json:

  "devDependencies": {
    "@types/jest": "^24.0.15",
    "@types/node": "^11.13.17",
    "jest": "^24.8.0",
    "ts-jest": "^24.0.2",
    "tslib": "^1.10.0",
    "tslint": "^5.18.0",
    "typescript": "^3.5.3"
  "dependencies": {
    "@types/flat": "0.0.28",
    "axios": "^0.18.1",
    "dotenv": "^7.0.0",
    "flat": "^4.1.0",
    "https-proxy-agent": "^3.0.1",
    "pg": "^7.11.0"

Let's analyze this on the example of axios package:

npm list | grep axios
├─┬ axios@0.18.1

$ npm info axios

axios@0.19.2 | MIT | deps: 1 | versions: 42
Promise based HTTP client for the browser and node.js

keywords: xhr, http, ajax, promise, node

.shasum: 3ea36c5d8818d0d5f8a8a97a6d36b86cdc00cb27

follow-redirects: 1.5.10 

- mzabriskie <>
- nickuraltsev <>

latest: 0.19.2       next: 0.19.0-beta.1  

published 3 months ago by emilyemorehouse <>

$ npm update axios

The last command does not do anything as we have:

"axios": "^0.18.1"

and as 0.x.x is special case, this means >=0.18.1 but < 0.19.0 (look here the rule: Semver cheatsheet).

So to update axios to the latest available version we can do:

$ npm uninstall axios
$ npm install axios

...after which in package.json we have:

"axios": "^0.19.2"

search search_terms...
Searches the registry for packages matching the search terms. [npm-search]

(short for run start)
npm runs the start script which is a command defined under scripts key in packages.json. This command usually starts the application with special configuration options (all listed in packages.json).

uninstalls a package, completely removing everything npm installed on its behalf.
-S, --save: Package will be removed from your dependencies.
-D, --save-dev: Package will be removed from your devDependencies.
-O, --save-optional: Package will be removed from your optionalDependencies.

If all packages are uninstalled package.json contains empty dependency list:

"dependencies": {}

uninstall -g package_name
uninstall --global package_name
Uninstalls globally installed package

Updates local packages to their latest versions allowed by the version specified with syntax ^ ("compatible with") or ~ ("reasonably close to") (see here for explanation in detail)  in package.json. Installs missing packages. Changes package.json and package-lock.json.
npm install vs. update - what's the difference?
How do I update devDependencies in NPM?

update -g 
Updates global packages.

view package_name
Shows data about a package. This can be used to check if some package actually exists as if package doesn't exist a message "404 Not found : package_name" is printed out. [npm-view]


$ npm view axios

axios@0.19.2 | MIT | deps: 1 | versions: 42
Promise based HTTP client for the browser and node.js

keywords: xhr, http, ajax, promise, node

.shasum: 3ea36c5d8818d0d5f8a8a97a6d36b86cdc00cb27
.integrity: sha512-fjgm5MvRHLhx+osE2xoekY70AhARk3a6hkN+3Io1jc00jtquGvxYlKlsFUhmUET0V5te6CcZI7lcv2Ym61mjHA==
.unpackedSize: 346.1 kB

follow-redirects: 1.5.10 

- mzabriskie <>
- nickuraltsev <>

latest: 0.19.2       next: 0.19.0-beta.1  

published 3 months ago by emilyemorehouse <>


    Products (tools, apps) used to come with a single package which contained both the core functionality and CLI.  Nowadays products are moving away from coupling the CLI and library together. The CLI now usually lives in the xxx-cli package.

    Installing package locally vs globally

    Sometimes it is better to install a package locally (project by project) then globally. There are couple of reasons for this:
    • Different projects on the same machine can depend on different versions of the package allowing you to update them individually.
    • Not having an implicit dependency on the environment you are working in makes your project far more portable and easier to setup.

    Where to install development dependencies?

    Shall development dependencies (e.g. Rollup, Webpack, Browserify, Gulp...) be installed globally or locally? Looking at reasons listed in the previous paragraph it makes more sense to install them locally. Benefits:
    • if required, different versions of these tools can be used in different projects 
    • all required dependencies can be installed in one go with npm install. There is no need for extra steps for installing packages globally (which, in turn, might also require root or admin privileges opening potential security holes...)

    How to run locally installed packages?

    • use npx to run their binaries directly (usually for some quick demonstrations, not in the real-world projects)
    • add them to scripts section of package.json and call them via npm run
    • add them to gulpfile and then Gulp takes care of executing them

    How to find quickly the version of some package installed on the local machine?

    >npm list dtrace-provider
    my-app@0.1.0 C:\dev\my-app
    `-- web-ext@2.9.1
    `-- bunyan@1.8.12
    `-- dtrace-provider@0.8.7

    Use -g to look for package installed globally on the machine.

    Find the version of an installed npm package

    How to move installed package from production to dev?

    Move a module from devDependencies to dependencies in npm package.json

    npm install <module_name> --save-prod
    npm install <module_name> --save-dev

    Test frameworks should be installed as dev dependencies.
    E.g. from Getting Started · Jest:

    npm install --save-dev jest

    TypeScript should (in standalone apps, not packages intended for distribution) be installed as dev dependencies.
    E.g. From DefinitelyTyped/DefinitelyTyped: The repository for high quality TypeScript type definitions.?:

    npm install --save-dev @types/node

    How to update all locally installed packages?

    From Updating packages downloaded from the registry | npm Documentation:

    We recommend regularly updating the local packages your project depends on to improve your code as improvements to its dependencies are made.

    From How to update all the Node dependencies to their latest version:

    Some of those updates are major releases. Running npm update won’t update the version of those. Major releases are never updated in this way because they (by definition) introduce breaking changes, and npm want to save you trouble.
    To update to a new major version all the packages, install the npm-check-updates package globally (...).

    Instead of upgrading all of them in bulk (which can introduce breaking changes if major version is increased in update) I'd play safely and update packages one by one.

    Common npm mistakes

    Introduction to Browserify

    Friday, 21 September 2018

    How to use "Insert JIRA Issue/Filter" macro in Confluence

    To add Insert JIRA Issue/Filter macro in Confluence follow the steps as in this image:

    When using this macro in Confluence, it is better to use filter ID rather than JQL. JQL con contain various values which can change in time e.g. the name of the fixVersion field or project name etc...If we change JQL string in JIRA we have to change it everywhere we used it in Confluence. If we use in Confluence filter ID (which is actually JQL query ID), the only place we have to make a change is JQL in JIRA - filter ID remains the same.

    Make sure that filter in JIRA is set to be shared e.g. with anyone logged-in. This will allow Confluence to execute filter in JIRA and display all JIRA tasks from it.

    Thursday, 20 September 2018

    Introduction to Gulp

    What is Gulp?

    Gulp is a:

    • toolkit for automating & enhancing JavaScript development workflow
    • Node.js based build system
    • JavaScript task runner which can automate common tasks (e.g. minification, error checking, bundling etc...)

    How to install Gulp?

    Two packages have to be installed: CLI and the core one.

    CLI package has to be installed globally:

    > npm install --global gulp-cli

    The local installation guarantees that the project build will not break even if system install is upgraded. The global gulp install (CLI) is there only to provide a binary command in the path. Gulp is designed in a way that a specific project can depend on a specific version of the build system so it will still build no matter the state or installed version of the global package.

    How to check if gulp CLI is installed?

    C:\dev\github\gulp-demo>npm list -g gulp-cli
    `-- gulp-cli@2.0.1

    Gulp core package has to be installed locally. To check if it's already been installed we can use:

    C:\dev\github\gulp-demo>npm list gulp
    gulp-demo@1.0.0 C:\dev\github\gulp-demo
    `-- (empty)

    Let's install it (but if package.json hasn't been created yet, execute npm init):

    C:\dev\github\gulp-demo>npm install --save-dev gulp
    npm WARN deprecated gulp-util@3.0.8: gulp-util is deprecated - replace it, following the guidelines at
    npm WARN deprecated graceful-fs@3.0.11: please upgrade to graceful-fs 4 for compatibility with current and future versions of Node.js
    npm WARN deprecated minimatch@2.0.10: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
    npm WARN deprecated minimatch@0.2.14: Please update to minimatch 3.0.2 or higher to avoid a RegExp DoS issue
    npm WARN deprecated graceful-fs@1.2.3: please upgrade to graceful-fs 4 for compatibility with current and future versions of Node.js
    npm notice created a lockfile as package-lock.json. You should commit this file.
    + gulp@3.9.1
    added 253 packages from 162 contributors and audited 1112 packages in 9.569s
    found 5 vulnerabilities (1 low, 4 high)
      run `npm audit fix` to fix them, or `npm audit` for details

    Let's verify installation:

    C:\dev\github\gulp-demo>npm list gulp
    gulp-demo@1.0.0 C:\dev\github\gulp-demo
    `-- gulp@3.9.1

    gulp package does not represent Gulp's latest version. [read here and here]
    In order to get the latest version we should use gulp@next.

    So let's uninstall version 3.9.1 and install the latest Gulp:

    C:\dev\github\gulp-demo>npm uninstall gulp
    removed 253 packages in 3.447s
    found 0 vulnerabilities

    C:\dev\github\gulp-demo>npm install --save-dev gulp@next
    npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.4 (node_modules\fsevents):
    npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.4: wanted {"os":"darwin","arch":"any"} (current: {"os":"win32","arch":"x64"})

    + gulp@4.0.0
    added 318 packages from 212 contributors and audited 5472 packages in 15.089s
    found 0 vulnerabilities

    How to check Gulp version?

    C:\dev\github\gulp-demo>gulp -v
    [13:49:01] CLI version 2.0.1
    [13:49:01] Local version 4.0.0

    How to configure Gulp?

    Let's try to run gulp now:

    [13:49:32] No gulpfile found

    Gulp requires a gulpfile.js file at the root of the project. Gulpfile contains tasks it can run from the command line. Let's define a default task which prints Hello world! to the console:


    var gulp = require('gulp');

    gulp.task('default', function() {
      // place code for your default task here
      console.log('Hello, world!');

    When we run gulp with no arguments, it will run the task named as default:

    [13:55:13] Using gulpfile C:\dev\github\gulp-demo\gulpfile.js
    [13:55:13] Starting 'default'...
    Hello, world!
    [13:55:13] The following tasks did not complete: default
    [13:55:13] Did you forget to signal async completion?

    We could have explicitly named the task we want to run:

    C:\dev\github\gulp-demo>gulp default
    [13:55:24] Using gulpfile C:\dev\github\gulp-demo\gulpfile.js
    [13:55:24] Starting 'default'...
    Hello, world!
    [13:55:24] The following tasks did not complete: default
    [13:55:24] Did you forget to signal async completion?

    Gulp CLI

    These are the Gulp CLI command line arguments:
    >gulp --help

    Usage: gulp [options] tasks

      --help, -h              Show this help.                              [boolean]
      --version, -v           Print the global and local gulp versions.    [boolean]
      --require               Will require a module before running the gulpfile.
                              This is useful for transpilers but also has other
                              applications.                                 [string]
      --gulpfile, -f          Manually set path of gulpfile. Useful if you have
                              multiple gulpfiles. This will set the CWD to the
                              gulpfile directory as well.                   [string]
      --cwd                   Manually set the CWD. The search for the gulpfile, as
                              well as the relativity of all requires will be from
                              here.                                         [string]
      --verify                Will verify plugins referenced in project's
                              package.json against the plugins blacklist.
      --tasks, -T             Print the task dependency tree for the loaded
                              gulpfile.                                    [boolean]
      --tasks-simple          Print a plaintext list of tasks for the loaded
                              gulpfile.                                    [boolean]
      --tasks-json            Print the task dependency tree, in JSON format, for
                              the loaded gulpfile.
      --tasks-depth, --depth  Specify the depth of the task dependency tree.[number]
      --compact-tasks         Reduce the output of task dependency tree by printing
                              only top tasks and their child tasks.        [boolean]
      --sort-tasks            Will sort top tasks of task dependency tree. [boolean]
      --color                 Will force gulp and gulp plugins to display colors,
                              even when no color support is detected.      [boolean]
      --no-color              Will force gulp and gulp plugins to not display
                              colors, even when color support is detected. [boolean]
      --silent, -S            Suppress all gulp logging.                   [boolean]
      --continue              Continue execution of tasks upon failure.    [boolean]
      --series                Run tasks given on the CLI in series (the default is
                              parallel).                                   [boolean]
      --log-level, -L         Set the loglevel. -L for least verbose and -LLLL for
                              most verbose. -LLL is default.                 [count]

    Lists all tasks defined in gulpfile.

    >gulp  --tasks
    [17:11:17] Tasks for C:\dev\gulp-demo\gulpfile.js
    [17:11:17] ├── build_brandA_environmentA
    [17:11:17] ├─┬ pack_brandA_environmentA
    [17:11:17] │ └─┬ <series>
    [17:11:17] │   ├── build_brandA_environmentA
    [17:11:17] │   └── <anonymous>
    [17:11:17] ├── build_brandB_environmentA
    [17:11:17] ├─┬ pack_brandB_environmentA
    [17:11:17] │ └─┬ <series>
    [17:11:17] │   ├── build_brandB_environmentA
    [17:11:17] │   └── <anonymous>
    [17:11:17] ├─┬ build
    [17:11:17] │ └─┬ <parallel>
    [17:11:17] │   ├── build_brandA_environmentA
    [17:11:17] │   └── build_brandB_environmentA
    [17:11:17] ├─┬ pack
    [17:11:17] │ └─┬ <parallel>
    [17:11:17] │   ├─┬ pack_brandA_environmentA
    [17:11:17] │   │ └─┬ <series>
    [17:11:17] │   │   ├── build_brandA_environmentA
    [17:11:17] │   │   └── <anonymous>
    [17:11:17] │   └─┬ pack_brandB_environmentA
    [17:11:17] │     └─┬ <series>
    [17:11:17] │       ├── build_brandB_environmentA
    [17:11:17] │       └── <anonymous>
    [17:11:17] ├─┬ watch
    [17:11:17] │ └─┬ <series>
    [17:11:17] │   ├─┬ build
    [17:11:17] │   │ └─┬ <parallel>
    [17:11:17] │   │   ├── build_brandA_environmentA
    [17:11:17] │   │   └── build_brandB_environmentA
    [17:11:17] │   └── watch
    [17:11:17] ├─┬ default
    [17:11:17] │ └─┬ <series>
    [17:11:17] │   └─┬ build
    [17:11:17] │     └─┬ <parallel>
    [17:11:17] │       ├── build_brandA_environmentA
    [17:11:17] │       └── build_brandB_environmentA
    [17:11:17] └── help

    Gulp API

    All function arguments can take arbitrary arguments.


       define a task
       gulp.task(task_name, function () {...})
       gulp.task(task_name,  gulp.series(task1_name, gulp.parallel(task2_name, task3_name)))


       Define which tasks or task and a function have to be executed in serial fashion (one after another). [source]
       gulp.series(task1_name, task2_name)
       gulp.series(task1_name, function() {...})
       gulp.series(task1_name, gulp.parallel(task2_name, task3_name))


       Define which tasks or task and a function have to be executed in parallel. [source]
       gulp.parallel(task1_name, task2_name)
       gulp.parallel(task1_name, function() {...})

       Define a function or task(s) to be executed each time any of files that matches the provided pattern change.
    Format:, function(){...}), gulp.series(task1_name, task2_name)), gulp.parallel(task1_name, task2_name))

    How is Gulp actually used in projects?

    Projects are usually npm-based so contain packages.json. Its "scripts" section contains one or more scripts which are basically executions of Gulp tasks (in form gulp [options] tasks). Gulp tasks are defined in gulpfile.js. Gulp is indirectly executed via npm.

    Further reading:

    Gulp - Getting Started
    Gulp js interview questions for beginners
    Gulp recipes
    How do I update to Gulp 4?
    A quick guide for switching to gulp 4